1. Field of the Invention
This invention relates generally to stable conductive elements and more specifically to stable conductive elements which resist chemical reactions and which are substantially impervious to passage of gases at elevated temperatures.
2. Description of the Prior Art
In the past, various compositions have been used for conductive elements for direct exposure to reactive agents over a range of temperatures. In general, such elements comprised mixtures or compounds including metals and semiconductors which would either degrade substantially on exposure to reaction gases at high temperatures or were so porous that undesired gases could cause contamination by passage through or entrapment in the heating element. Thus in general such conductive elements have had to be separated from the working space of the furnace or reaction chamber by a non-porous non-reactive coating or by a tube or other enclosure to separate the conductive means from the working space. Yet another problem exists with conductive elements exposed directly to reactants in that eventually cleaning or etching is required to eliminate build up of reaction products, for example by chemical etching. Inability of the conductive element to withstand the cleaning method would lead to its premature destruction. For example, it is known to use R.F. heated susceptors fabricated from graphite, which is both reactive and permeable. To isolate such a susceptor from the reacton ambient, it is known to provide a coating such as silicon carbide. Such a coating is easily damaged, and tends to crack due to thermal excursions. Similarly, where freedom from unwanted reactants is required in a furnace, the workpieces must be isolated from heating elements, such as resistance wires or silicon carbide resistance elements, by a furnace tube constructed, for example, from quartz or alumina, which are expensive, prone to breakage, difficult to clean, and introduce undesirable thermal resistance between the heating elements and the workpieces.
Thus, a need existed to provide more inert, less pervious, and stronger conductive elements which could be in direct contact with the working space. In the case of a heated chamber, a need existed to provide a heating element which also serves to isolate the working space from ambient reactants.